Abstract
More than any technology on the horizon today, process fieldbus will have the most impact on the way we look at control systems and will forever change the dynamics of the process control and instrumentation marketplace. First conceived as a simple digital replacement for 4-20mA communications, the concept of fieldbus was hastened by the introduction of smart field devices in the 1980s. These devices boasted on-board microprocessors and remote communications that allowed more data than ever before to be transmitted from the field. Conventional analog communications, however, created a bottleneck that could only be resolved through the use of digital communications.
The Evolution of Fieldbus
Primary Fieldbus Players
The SP50 committee of the Instrument Society of America (ISA) was the original focal point for fieldbus development. The International Electrotechnical Commission (IEC) is the main global organization with responsibility for definition of an international fieldbus standard. The ISA/SP50 and IEC committees meet jointly to facilitate the development of an international standard. ISA SP50 was the same committee that introduced the 4-20mA standard back in the 1970s.
The process of specifying both the IEC and ISA SP50 protocol remains unfinished, in spite of the fact that the committee was reconvened to begin the task as far back as 1984. Infighting among competing protocols contributed to the delay, but the decision to develop new technology instead of the typical sanctioning of an existing option added to the dilemma. Much of this infighting was between control system suppliers and field device suppliers as the potential for fieldbus to realign the balance of power in the process industries in favor of the field device suppliers became apparent.
Delays in the standardization process led to the rise of competitors such as the now-defunct Interoperable Systems Project (ISP) and WorldFIP North America, which combined to form the Fieldbus Foundation. The Profibus User Organization (PNO) took the remaining technology from the ISP development effort and combined it with Profibus DP to create the Profibus PA protocol. ARC views Profibus PA and Foundation Fieldbus as the leading process fieldbus contenders in the world today.
The physical layer of both the IEC and ISA/SP50 fieldbus standard, or layer 1, is already defined. Both the data link layer and application layer have yet to be agreed upon. The scope of the IEC and ISA/SP50 fieldbus function blocks is still being defined, and the draft on functional requirements is being written and edited. The draft specification for the function blocks themselves is in the initial stages of development. Given the current state of protocol development among both of these standards organizations, ARC believes that it will be years before either the IEC and ISA/SP50 committees define a complete fieldbus standard. (see Figures 1 through 4)
Fieldbus Foundation
The Fieldbus Foundation (FF) was formed in 1994 when two rival fieldbus groups, the Interoperable Systems Project and WorldFIP North America, agreed to merge their resources. The stated objective of the Fieldbus Foundation is to develop a single, interoperable fieldbus protocol as quickly as possible.
The protocol being pursued by the FF is a subset of the SP50 protocol. The FF is now working closely with the SP50 committee to complete development of a fieldbus standard as soon as possible. FF is an independent organization comprised of over 100 control and instrumentation suppliers. All of the leading control system and field instrument suppliers worldwide are members of the FF.
The FF released the technical specification for the low-speed H1 FF fieldbus communication stack and user layer in the fourth quarter of 1996. Device Description (DD) technology, which provides interoperability between devices from different suppliers, was also released in late 1996.
The FF recently made conformance and interoperability testing software kits available to product developers. Both the conformance and interoperability test kits were developed for the FF by the Fraunhofer Institute — a designer of network and communication testing software. Conformance testing ensures that fieldbus products conform to the communication stack and technology standards laid out by the FF. Interoperability certification ensures that any Foundation Fieldbus-compatible product will provide a certain level of interoperability with other products from different suppliers. The Foundation is handling the initial round of interoperability tests for the first wave of FF-compatible devices. All devices submitted to the FF for interoperability testing must first pass FF conformance testing.
Only products that have met these conformance and interoperability standards can be given the stamp of approval by the Fieldbus Foundation. Lack of conformance and interoperability testing, however, does not prevent suppliers from selling products incorporating FF technology. Many suppliers are already selling FF-compatible products that have not gone through the full range of testing from the Foundation.
Since Foundation Fieldbus is an open technology, anybody with enough money can purchase the communication stack, chips and other components needed to manufacture Foundation Fieldbus products. This does not mean, however, that these products meet the conformance and interoperability standards of the FF. Some suppliers are already selling products that contain the FF communications stack, but have not gone through both conformance and interoperability testing. ARC recommends that any user purchasing Foundation Fieldbus products make sure that the product has the official seal of approval from the FF, indicating that the product has passed interoperability and conformance testing.
Some users, however, may choose to work closely with a single vendor on a pilot project before interoperability and conformance testing can be completed. ARC recommends that the user get a commitment from the supplier that they will have their products certified by the Fieldbus Foundation as soon as a complete set of conformance and interoperability tools are available. (See Figures 5 Through 8).
Continued in Part 2: Profibus PA